In recent years, low-NOx high-efficiency combustion, such as fully-premixed combustion, has been promoted, and it is required to accurately control the supply amount of fuel gas and air (hereinafter referred to as gas) to be supplied to, for example, a gas burner or a gas engine. To achieve such control, it is important to detect the flow rate of the gas (that means fuel gas or combustion air) supplied to a gas burner and the like with high accuracy.
For a flow detector for detecting the flow rate of gas (fluid), for example, Unexamined Japanese Patent Application No. 4-230808 discloses one in which a pair of temperature sensors arranged with/without a heater therebetween in the gas-flowing direction are provided onto the inner wall of a main path to be exposed. This thermal flowmeter detects temperature distribution that is changed by the flow velocity of the gas by a temperature difference detected by the temperature sensors, to thereby measure the mass flow rate of the gas according to the temperature difference. In the thermal flowmeter, however, a heater and the temperature sensors are brought into direct contact with the gas running through the main path. Therefore, the thermal flowmeter is not suitable for the flow rate measurement of the gas having a high temperature of, for example about 300 degrees centigrade, in respect of heat resistance. Moreover, in the thermal flowmeter, a detection output with respect to an increase in gas flow shows a curved alteration characteristic, so that an area in which the flow and the detection output are considered to be in a proportional relation is narrow. For this reason, if the flow is to be detected in a relatively wide area, calculation for converting the detection output showing the curved alteration characteristic with respect to the flow into a linear alteration characteristic is required.
Disclosed in, for example, Unexamined Japanese Patent Application No. 10-307047 is an orifice-type flowmeter in which an orifice (throttle) is disposed in a path, to thereby detect a gas flow rate by pressure (differential pressure) detected through the orifice. This orifice-type flowmeter is so constructed as to shunt part of the fluid flowing through the main path into a branch path. Therefore, even a high-temperature gas can be detected after being refrigerated in the branch path. On the other hand, in the orifice-type flowmeter, a throttle ratio of the gas path which is obtained by the orifice needs to fall in the range of from about 0.1 to 0.8, which is surely accompanied by pressure loss.
Furthermore, it is necessary that an inflow-side opening end portion of the branch path be disposed on the upper stream side of the orifice, and an outflow-side opening end portion be disposed on the lower stream side separately. As a result, it is certain that the inflow-side opening end portion and the outflow-side opening end portion are positioned apart from each other at some distance in the longitudinal direction. Accordingly, in case that there generates oscillation in the gas flow in the main path due to combustion or the like, the oscillation occasionally cannot be detected by the orifice-type flowmeter. In other words, the orifice-type flowmeter has the disadvantage that oscillation at a specific frequency corresponding to the distance between the inflow-side opening end portion and the outflow-side opening end portion cannot be detected. This phenomenon is attributable to the fact that the pressure of the inflow-side opening end portion and that of the outflow-side opening end portion are equalized, so that there generates no flow in the branch path.